In recent years, the applications of FRPs have broadened and they have begun to be widely applied not just in the aerospace and sports fields but also to large-size structures in fields such as civil engineering and construction.
In particular, the use of FRPs in the repair or reinforcement of concrete structures is the subject of attention and is increasing for reasons such as the easing of vehicle weight restrictions, the occurrence of major earthquakes and the ease of use of such FRPs. Concrete structures include floor slabs, bridge piers, tunnels and buildings, but there occurs rusting of the internal reinforcements due to neutralization of the concrete and salt damage, and deterioration due to the alkali aggregate reaction, and these constitute problems for society. Again, when vibration due to passing traffic or due to earthquakes, and the pressure of earth and sand in the case of a tunnel, are also added, the cracks which occur in the concrete are widened and the progress of deterioration hastened. Moreover, since structures in the civil engineering and construction fields are often of a large size, it is not possible to predict rupture and there is the possibility of a major accident arising due to sudden failure.
Hence, a technique is demanded for observing the fatigue of structures and the progress of deterioration, and for predicting failure of a structure prior to the event. However, non-destructive inspection such as direct observation is mainly employed at present, and it is not easy to ascertain the state of fatigue or deterioration correctly.
Strain gauges have long been known as a means for detecting strain. However, since they detect strain within their own area and, moreover, since their length is short at no more than 30 mm, they can only detect local strain. Consequently, it is necessary to affix numerous strain gauges to detect strain widely in a large-size structure.
On the other hand, in Japanese Unexamined (Kokai) Patent Publication No. 60-114741, there is described a method in which carbon fiber filament yarn is arranged within an FRP member and the extent of damage to the carbon filaments from which the yarn is composed is measured from the change in the resistance of said yarn, and a lowering of the rigidity of the member or fatigue failure thereby detected beforehand. By means of this method, it is possible to detect strain in large-size structures over a broad area.
However, the numerous carbon fiber filaments from which the yarn is composed are present within the yarn in various states of alignment, and the aligned state thereof will differ between yarns. Hence, the extent of filament failure under an identical load will differ according to the particular yarn, and the reproducibility of changes in electrical resistance in these yarns is poor. Furthermore, with regard to the terminals at the two ends of the yarns where resistance measurement is required, it is necessary that there be contact with all the individual filaments, but it is difficult to effect contact with all the thousands or tens of thousands of filaments of diameter just a few microns within the yarn.
In Japanese Unexamined (Kokai) Patent Publication No. 2-38945, there is described a method for detecting fatigue failure in which a metal wire is arranged within a structure comprising a glass fiber reinforced composite material, and the change in electrical resistance thereof measured.
However, in the case where a metal wire is arranged within an FRP used for the repair/reinforcement of a large size structure, in particular a concrete structure, the surface of the structure is not restricted to being level and it is often curved or a surface with indentations/projections. Thus, in the fabrication of the FRP, the fiber reinforcing material is affixed running along the surface of the structure and resin impregnation performed at the same time. Consequently, the arrangement of the metal wire is a complex procedure which is carried out by hand on the reinforcing fiber base material while the resin remains uncured immediately after the impregnation. By such a method of arrangement, very often the metal wire is not arranged parallel to the reinforcing wire but meanders, so strain in the FRP which has been designed based on loads and strain in the reinforcing fiber direction is not accurately detected from said metal wire.
Now, in the carbon fiber reinforced plastics field, structures such as beams for aircraft or the like are produced by layup of numerous layers of sheet-shaped carbon fiber base material (for example prepreg wherein carbon fiber is arranged uniaxially in parallel and which has been impregnated with B-stage epoxy resin) such that the fiber is orientated in the desired direction, after which the resin is cured using an autoclave. However, the setting of the lamination direction and the number of plies of sheet-shaped carbon fiber base material in the laminate is carried out manually, so there is the possibility of mistakes being made. Hence, in the case of, for example, the beams which constitute the primary structural bodies of aircraft, samples are sometimes cut from the ends of the beam, the resin then burnt away and the lamination direction and the number of laminated plies in the remaining carbon fiber checked.
Moreover, in the case where repair or reinforcement of a concrete structure is carried out by affixing sheet-shaped carbon fiber base material to the structure, then impregnating with a cold-curing type epoxy resin and curing, the checking of the number of plies in the laminate of sheet-shaped base material is carried out for example by the method of taking photographs following the completion of the layup of each single layer, but this is extremely troublesome.